1. Field of the Invention
The present invention relates to a device for measuring combustible-gas concentration and a method for measuring combustible-gas concentration by use of the device. More particularly, the invention relates to such devices and methods capable of measuring the concentration of a combustible gas such as hydrocarbon gas contained in an exhaust gas emitted from a lean-burn engine including a diesel engine.
The present device enables measurement of the combustible gas concentration with low dependency on the high concentration of oxygen contained in such a lean-burn engine.
2. Description of the Related Art
Conventionally, in order to optimize the combustion efficiency of an internal combustion engine and to maximize combustion of combustible gases contained in a combustion exhaust gas, a percentage of a gas component, particularly oxygen, contained in the combustion exhaust gas is measured, and the measured data is fed back to an engine control system. However, in order to cope with a trend toward tightening of exhaust gas regulations and a goal of attaining fuel economy in recent years, there have been demands for more sensitive detection of a combustible gas component contained in the exhaust gas in addition to apparatus and methods for accurately measuring concentration of the combustible gas component.
For example, since the lean-bum engine can promote fuel economy but emits a large amount of nitrogen oxides, a system has been studied as shown in FIG. 1 for eliminating nitrogen oxides, in which system the fuel is added into the exhaust gas by an injector placed at a position upstream of a catalytic converter. Also, there have been demands for attachment of a catalyst to the diesel engine that is conventionally unequipped with a catalytic converter. Thus, a sensor system as shown in FIGS. 2 or 3 has been studied, which is capable of measuring the concentration of a combustible gas and/or detecting deterioration of the catalytic converter used therein.
However, a lean-bum engine involves great variations in oxygen concentration as compared to a conventional stoichiometricly controlled engine and exhibits a particularly high oxygen concentration under lean conditions. Thus, there have been demands for a device for measuring combustible-gas concentration capable of detecting a hydrocarbon that contains a relatively large number of carbon atoms (about 2-15 carbon atoms), without dependence on oxygen concentration.
Techniques for detecting such a combustible gas are disclosed in PCT Application Laid-open No. WO 95/25277 and Japanese Patent Application Laid-Open (kokai) No. 82763/1998. According to the former publication, a detection electrode may contain gold, silver, platinum, or bismuth. The latter publication does not particularly describe a detection electrode. These publications do not mention techniques that decrease a measurement problem such as dependence on oxygen concentration and/or dependence on temperature of the exhaust gas.
It is therefore an object of the present invention to solve the above-mentioned problems of the prior art, and to provide a device for measuring combustible-gas concentration and a method for measuring combustible-gas concentration using of the same. This device and method enables sensitive detection of a combustible gas such as a hydrocarbon gas, and accurate detection of concentration of such a combustible gas while suppressing dependence on oxygen concentration in a combustion exhaust gas and/or dependence on temperature of a gas to be measured, and as well exhibiting a stable response.
A device of a first aspect of the invention for measuring combustible-gas concentration comprises an oxygen-ion-conductive solid electrolyte element, a reference electrode and a detection electrode formed on the surface of the solid electrolyte element, and a heater element for heating the solid electrolyte element. The device is characterized in that the detection electrode contains gold and a metallic oxide. The xe2x80x9cmetallic oxidexe2x80x9d may be an oxide of a transition metal. Preferably, the detection electrode contains an oxide of at least one element selected from among In, Fe, Ta, Ga, Sr, Eu, W, Ce, Ti, Zr and Sn. Most preferably, the detection electrode contains one of indium oxide and iron oxide. The metallic oxide greatly reduces dependence on oxygen concentration of the device for measuring combustible-gas concentration, according to the invention.
Examples of the above-mentioned xe2x80x9ccombustible gasxe2x80x9d include hydrocarbons contains 2-15 carbon atoms, hydrogen, carbon monoxide and ammonia. The above-mentioned xe2x80x9csolid electrolyte elementxe2x80x9d may be a known oxygen-ion-conductive solid electrolyte body such as a zirconia-based ceramic body and an LaGaO3-based ceramic body.
The above-mentioned xe2x80x9creference electrodexe2x80x9d is an electrode which is brought into contact with a reference gas or an oxygen atmosphere of constant pressure established by means of an oxygen pump. The reference electrode shows a higher electric potential than does a detection electrode upon contact with a combustible gas component contained in the gas being measured.
The above-mentioned xe2x80x9cdetection electrodexe2x80x9d is an electrode exposed to the gas to be measured.
The above-mentioned xe2x80x9cheater elementxe2x80x9d is an element for heating the solid electrolyte element so as to maintain the solid electrolyte element at a predetermined temperature, and may assume a conventional form and may be formed from a known material. In the case where the above-mentioned xe2x80x9csolid electrolyte elementxe2x80x9d assumes the form of a closed-bottomed cylinder with a hollow inside, the heater element may assume a rod form, such as the form of a round bar or strap, so as to be placed in the hollow . In the case where the device for measuring combustible-gas concentration assumes the form of a laminate, the heater element may be an integral portion of the laminate or may be disposed separately in the vicinity of the solid electrolyte element.
A device of a second aspect of the invention for measuring combustible-gas concentration is characterized in that the detection electrode comprises a first electrode layer formed on the surface of the solid electrolyte element and a second electrode layer formed on the surface of the first electrode layer; the first electrode layer containing at least one of platinum and gold; and the second electrode layer containing at least one of gold and a metallic oxide (except for a case where the first electrode layer is identical in composition with the second electrode layer).
The above-mentioned xe2x80x9ccombustible gas,xe2x80x9d xe2x80x9creference electrode,xe2x80x9d xe2x80x9cdetection electrode,xe2x80x9d xe2x80x9cheater element,xe2x80x9d xe2x80x9csolid electrolyte element,xe2x80x9d and xe2x80x9cmetallic oxidexe2x80x9d are similar to those of the first aspect of the invention. By employing the detection electrode structure comprising the xe2x80x9cfirst electrode layerxe2x80x9d and the xe2x80x9csecond electrode layer,xe2x80x9d sensor output can be reduced (or rather lowered) for a gas to be measured which does not contain a combustible gas (hereinafter, this sensor output is called merely xe2x80x9coffsetxe2x80x9d). Also, adhesion of the detection electrode to the solid electrolyte element can be improved in this structure.
The offset is apt to increase when an electrode contains a metallic oxide is formed in direct contact with the solid electrolyte element. An increase in the offset is not preferred, since the sensor output relatively decreases for a combustible gas contained in the gas to be measured. By contrast according to the invention, by forming the first electrode layer containing at least one of platinum and gold, between the solid electrolyte element and the second electrode layer that contains a metallic oxide, the offset can be suppressed to a low level such that a combustible gas can be detected and measured more accurately.
By employing a detection electrode contains gold and a metallic oxide, a device for measuring combustible-gas concentration can suppress offset to 0-40 mV and further to 0-10 mV. The device for measuring combustible-gas concentration equipped with the detection electrode which contains gold and a metallic oxide is 40-120 mV lower in offset than one equipped with a detection electrode which contains only gold.
When an electrode contains a metallic oxide is formed in contact with the solid electrolyte element, adhesion between the solid electrolyte element and the detection electrode may become insufficient. By contrast, by forming the first electrode layer, which contains at least one of platinum and gold, between the solid electrolyte element and the second electrode layer, which contains a metallic oxide, adhesion between the solid electrolyte element and the detection electrode can be greatly improved.
In order to further improve the adhesion, a thin layer (thickness: 0.01-30 xcexcm) of plated gold or a thin layer (thickness: 0.01-30 xcexcm) of a baked organic gold compound may be formed between the first electrode layer and the second electrode layer. Also, by forming a similar thin layer between the solid electrolyte element and the first electrode layer, an adhesive layer can be improved.
When the amount of the first electrode layer is taken as 100 parts by weight (hereinafter, xe2x80x9cpartsxe2x80x9d), the first electrode layer preferably contains platinum or gold in an amount of not less than 50 parts, more preferably not less than 80 parts, further preferably not less than 90 parts. The first electrode layer may be formed from only platinum or gold. The first electrode layer may contain rhodium or indium in addition to platinum or gold. These elements improve heat resistance of the first electrode layer. For example, the first electrode layer may contain 90 parts of platinum and 10 parts of rhodium.
The thickness of the first electrode layer is not particularly limited, but is preferably not greater than 25 xcexcm, more preferably not greater than 5 xcexcm.
A device of a third aspect of the invention for measuring combustible-gas concentration is characterized in that the detection electrode comprises a first electrode layer formed on the surface of the solid electrolyte element, a second electrode layer formed on the surface of the first electrode layer, and a third electrode layer formed on the surface of the second electrode; the first electrode layer containing a predominant amount of platinum; the second electrode layer containing a predominant amount of gold; and the third electrode layer containing a metallic oxide and optionally gold.
The above-mentioned xe2x80x9ccombustible gas,xe2x80x9d xe2x80x9creference electrode,xe2x80x9d xe2x80x9cdetection electrode,xe2x80x9d xe2x80x9cheater element,xe2x80x9d xe2x80x9csolid electrolyte element,xe2x80x9d and xe2x80x9cmetallic oxidexe2x80x9d are similar to those of the first aspect of the invention. When the amount of the above-mentioned xe2x80x9cfirst electrode layerxe2x80x9d is taken as 100 parts, the first electrode layer preferably contains platinum in an amount of not less than 80 parts (more preferably not less than 90 parts, further preferably not less than 95 parts). When the amount of the above-mentioned xe2x80x9csecond electrode layerxe2x80x9d is taken as 100 parts, the second electrode layer preferably contains gold in an amount of not less than 80 parts (more preferably not less than 90 parts, further preferably not less than 95 parts).
By employing the detection electrode comprising the xe2x80x9cfirst electrode layer,xe2x80x9d the xe2x80x9csecond electrode layer,xe2x80x9d and the xe2x80x9cthird electrode layer,xe2x80x9d offset can be reduced as in the case of the second aspect of the invention. Also, adhesion between the detection electrode and the solid electrolyte element can be further improved. As in the case of the second aspect of the invention, a thin layer of plated gold or a thin layer of a baked organic gold compound may be formed between the electrodes so as to improve adhesion between the detection electrode and the solid electrolyte element.
The detection electrode in the first through third aspects of the invention may be formed on the entire portion of the surface of the solid electrolyte element which is exposed to a gas to be measured. However, preferably, as specified in a fourth aspect of the invention, the detection electrode is formed on the surface of the solid electrolyte element at only a portion corresponding to a heating resistor formed within the heater element (hereinafter, this portion is called a xe2x80x9cuniformly heated high-temperature portionxe2x80x9d). The xe2x80x9cuniformly heated high-temperature portionxe2x80x9d is a portion of the surface of the solid electrolyte element under which the heating resistor is present.
Preferably, as specified in a fifth aspect of the invention, the detection electrode is formed on the surface of the solid electrolyte element at only a portion extending from an end portion of the solid electrolyte element to the vicinity of the interface between a heating resistor and a heating-resistor lead portion, which are formed within the heater element (hereinafter, this portion is also called a xe2x80x9cuniformly heated high-temperature portionxe2x80x9d).
When the device for measuring combustible-gas concentration comprises a solid electrolyte element having the form of a closed-bottomed cylinder, the portion extending between the xe2x80x9cinterface between a heating resistor and a heating-resistor lead portionxe2x80x9d and the xe2x80x9cend portion of the solid electrolyte elementxe2x80x9d is a portion enclosed by B and T shown in FIGS. 5 and 6. When the device for measuring combustible-gas concentration comprises a solid electrolyte element of a laminate form, the portion extending between the xe2x80x9cinterface between a heating resistor and a heating-resistor lead portionxe2x80x9d and the xe2x80x9cend portion of the solid electrolyte elementxe2x80x9d is a portion enclosed by B and T shown in FIG. 7.
The uniformly heated high-temperature portion in the fourth and fifth aspects of the invention is where the surface temperature of the solid electrolyte element can easily be maintained at higher temperature in a uniform, stable manner by means of the heater element. Accordingly, by forming the electrodes on only this uniformly heated high-temperature portion, temperature dependence of sensor output can be suppressed to a low level. The surface temperature of the solid electrolyte element as measured at this uniformly heated high-temperature portion is maintained preferably at 350-750xc2x0 C. (more preferably 450-650xc2x0 C., further preferably 500-600xc2x0 C.). Further, the surface temperature of the uniformly heated high-temperature portion can be controlled preferably to a set temperature xc2x150xc2x0 C. (more preferably a set temperature +30xc2x0 C., further preferably a set temperature xc2x110xc2x0 C.).
In the case of a solid electrolyte element having the form of a closed-bottomed cylinder, for example, when the interface between the heating resistor and the heating-resistor lead portion is represented by B shown in FIGS. 5 and 6, this uniformly heated high-temperature portion is a portion enclosed by a plane which is located in parallel with the interface within a 5 mm range (preferably within a 2 mm range) with respect to the interface, and plane T which is in parallel with the interface and includes an end portion of the solid electrolyte element.
In the case of a solid electrolyte element of a laminate form, when the interface between the heating resistor and the heating-resistor lead portion is represented by B shown in FIG. 7, this uniformly heated high-temperature portion is a portion enclosed by a plane which is located in parallel with the interface within a 3 mm range (preferably within a 1 mm range) with respect to the interface, and a plane which is in parallel with the interface and includes an end portion of the solid electrolyte element. Notably, the detection element may be formed on the entire surface of the uniformly heated high-temperature portion or on a portion of the surface of the uniformly heated high-temperature portion. Preferably, the reference electrode is formed on or in a portion of the uniformly heated high-temperature portion.
A device of a sixth aspect of the invention for measuring combustible-gas concentration is characterized in that the detection electrode contains gold and optionally platinum and is formed on the solid electrolyte element at only a portion extending from an end portion of the solid electrolyte element to the vicinity of the interface between a heating resistor and a heating-resistor lead portion, which are formed within the heater element.
The above-mentioned xe2x80x9ccombustible gas,xe2x80x9d xe2x80x9creference electrode,xe2x80x9d xe2x80x9cdetection electrode,xe2x80x9d xe2x80x9cheater element,xe2x80x9d xe2x80x9csolid electrolyte element,xe2x80x9d and xe2x80x9cmetallic oxidexe2x80x9d are similar to those of the first aspect of the invention. The above-mentioned xe2x80x9cvicinity of the interface between a heating resistor and a heating-resistor lead portionxe2x80x9d and xe2x80x9cend portion of the solid electrolyte elementxe2x80x9d are similar to those of the fourth aspect of the invention.
In the devices of the first through sixth aspects of the invention for measuring combustible-gas concentration, when the first electrode layer contains platinum (preferably not less than 50 parts, more preferably not less than 80 parts) and the second electrode layer contains gold, it is preferable that the second electrode layer be formed from paste which contains gold powder of an average grain size of 0.1-100 xcexcm (preferably 0.2-50 xcexcm, more preferably 0.5-30 xcexcm), by baking the paste. When the average grain size of gold powder is less than 0.1 xcexcm, sufficient sensitivity is not obtained. When the average grain size is in excess of 100 xcexcm, the second electrode layer fails to have a uniform thickness, potentially causing variations in sensor output.
A device of a seventh aspect of the invention for measuring combustible-gas concentration is characterized in that the detection electrode comprises a first electrode layer formed on the surface of the solid electrolyte element and a second electrode layer formed on the surface of the first electrode layer; the first electrode layer containing platinum; the second electrode layer containing gold (except for a case where the first electrode layer is identical in composition with the second electrode layer); and the second electrode layer is formed from paste containing gold powder of an average grain size of 1-100 xcexcm (preferably 2-50 xcexcm, more preferably 5-30 xcexcm) and by baking the paste.
The above-mentioned xe2x80x9ccombustible gas,xe2x80x9d xe2x80x9creference electrode,xe2x80x9d xe2x80x9cdetection electrode,xe2x80x9d xe2x80x9cheater element,xe2x80x9d and xe2x80x9csolid electrolyte elementxe2x80x9d are similar to those of the first aspect of the invention. When the average grain size of gold powder is less than 1 xcexcm, sufficient improvement in sensitivity is not attained. When the average grain size is in excess of 100 xcexcm, the second electrode layer fails to have a uniform thickness, potentially causing variations in sensor output. By forming the detection electrode from gold powder of such a large grain size, sensitivity can be improved in detecting a combustible gas. The specific reason why sensitivity is improved is not definitely known. Gold powder of a large grain size tends to maintain a large grain size even after being subjected to baking. It is considered that gold contained in the detection electrode is preferably present while maintaining a large grain size even after baking.
As in the case of the first aspect of the invention, the second electrode layer may contain a metallic oxide in addition to gold. When the second electrode layer contains a predominant amount of gold and a third electrode layer is formed on the surface of the second electrode layer, the third electrode layer may contain gold and a metallic oxide. Further, when the first electrode layer contains gold and the third electrode layer contains gold, it is preferable that the first and third electrode layers be formed from respective pastes which contain gold powder of similar grain sizes.
Preferably, as specified in an eighth aspect of the invention, a diffusion layer is formed on the surface of the detection electrode in any of the first through seventh aspects of the invention. The above-mentioned xe2x80x9cdiffusion layerxe2x80x9d is not particularly limited so long as it permits a combustible gas contained in a gas to be measured to reach the detection electrode. The diffusion layer is conventionally formed from spinel and/or alumina. Passing through this diffusion layer, a gas to be measured reaches the detection electrode at a substantially constant velocity or flow rate, regardless of velocity or flow rate at which the gas to be measured reaches the surface of the diffusion layer. That is, dependence on velocity or flow rate of a gas to be measured can be reduced. The diffusion layer may also serve as a protection layer against poisoning and a reinforcement layer for increasing strength. The diffusion layer may be formed of two or more layers.
Preferably, as specified in a ninth aspect of the invention, the diffusion layer contains a component which oxidizes at least one of hydrogen and carbon monoxide. The device of the present aspect of the invention for measuring combustible-gas concentration may be particularly sensitive to hydrogen and in some cases carbon monoxide among combustible gases and may provide sensor output accordingly. In order to obtain sensor output which excludes a detection signal associated with at least one of hydrogen and carbon monoxide, hydrogen and/or carbon monoxide is oxidized or adsorbed so as to prevent the same from reaching the detection electrode. Thus, combustible gases excluding hydrogen and/or carbon monoxide can be accurately detected and measured. Examples of a catalytic component for oxidizing hydrogen include Pb, Ag, Au, Pd, Pt, Ir, Ru, Rh, Co, Ni, Mn, Cu, Cd, Fe, V, Cr, Ce, Y and La. In order to obtain sensor output which excludes a detection signal associated with carbon monoxide, the diffusion layer may contain a component capable of selectively or preferentially oxidizing or adsorbing carbon monoxide. Examples of such a component include Pb, Ag, Au, Pd, Pt, Ir, Ru, Rh, Co, Ni, Mn, Cu, Cd, Fe, V, Cr, Ce, Y and La. Notably, these components contained in the diffusion layer oxidize nitrogen monoxide and reduce other nitrogen oxides.
Particularly preferably, as specified in a tenth aspect of the invention, the diffusion layer contains at least one of platinum and palladium. When the diffusion layer contains xe2x80x9cplatinum,xe2x80x9d sensor output for hydrogen and particularly carbon monoxide can be reduced as compared with the case where the diffusion layer does not contain platinum. In this case, sensor output for combustible gases other than carbon monoxide also decreases. However, since sensor output for hydrogen and carbon monoxidexe2x80x94which would otherwise be detected with particular sensitivityxe2x80x94is decreased, sensitivity for combustible gases other than hydrogen and carbon monoxide relatively increases. Thus, even when a gas to be measured contains hydrogen and carbon monoxide, their effect on measurement can be suppressed to a small degree, whereby combustible gases other than hydrogen and carbon monoxide can be accurately detected and measured.
When the device for measuring combustible-gas concentration in which the diffusion layer contains platinum is applied to measurement of a gas to be measured which contains hydrogen; specifically, a gas to be measured which contains H2 (1000 ppm), O2 (7%), CO2 (10%), H2O (10%), and N2 (balance) and which has a temperature of 300xc2x0 C. and flows at 15 liters/min, and the device is controlled in the course of measurement such that the surface temperature of the detection electrode becomes 570xc2x0 C., sensor output for hydrogen can be reduced by 80 mV or more (approx. 25% or more) as compared with measurement by a device in which the diffusion layer does not contain platinum.
When the device for measuring combustible-gas concentration in which the diffusion layer contains platinum is applied to measurement of a gas to be measured which contains carbon monoxide; specifically, a gas to be measured which contains CO (1000 ppm), O2 (7%), CO2 (10%), H2O (10%), and N2 (balance) and which has a temperature of 300xc2x0 C. and flows at 15 liters/min, and the device is controlled in the course of measurement such that the surface temperature of the detection electrode becomes 570xc2x0 C., sensor output for carbon monoxide can be reduced by 80 mV or more (approx. 80% or more), further by 85 mV or more (approx. 85% or more), as compared with measurement by a device in which the diffusion layer does not contain platinum.
When the diffusion layer contains xe2x80x9cpalladium,xe2x80x9d sensor output for combustible gases other than carbon monoxide can be increased as compared with the case where the diffusion layer does not contain palladium. At the same time, sensitivity for carbon monoxide can be reduced. Accordingly, while the effect of carbon monoxidexe2x80x94which would otherwise be detected with particular sensitivityxe2x80x94on measurement is suppressed to a small degree, combustible gases other than carbon monoxide can be accurately detected and measured.
When the device for measuring combustible-gas concentration in which the diffusion layer contains palladium is applied to measurement of a gas to be measured which contains carbon monoxide; specifically, a gas to be measured which contains propylene (1000 ppm) or CO (1000 ppm), O2 (7%), CO2 (10%), H2O (10%), and N2 (balance) and which has a temperature of 300xc2x0 C. and flows at 15 liters/min, and the device is controlled in the course of measurement such that the surface temperature of the detection electrode becomes 570xc2x0 C., sensor output for carbon monoxide can be reduced by 14 mV or more (approx. 15% or more), further by 20 mV or more (approx. 20% or more), as compared with measurement by a device in which the diffusion layer does not contain palladium. Further, this device for measuring combustible-gas concentration can improve sensor output for propylene by 15 mV or more (approx. 13% or more), further by 20 mV or more (approx. 16% or more).
Preferably, as specified in an eleventh aspect of the invention, the device of the present invention for measuring combustible-gas concentration further comprises temperature control means for controlling voltage applied to the heater element on the basis of the internal resistance of the solid electrolyte element. The structure of the above-mentioned xe2x80x9ctemperature control meansxe2x80x9d is not particularly limited. Typically, the temperature control means comprises means for measuring the internal resistance of the solid electrolyte element and applied-voltage control means for controlling voltage applied to the heater element on the basis of the internal resistance. This internal resistance can be obtained by measuring resistance between the reference electrode and the detection electrode, which are formed on the solid electrolyte element. Alternatively, another conductive layer serving as an internal-resistance detection electrode may be formed on the solid electrolyte element, and the resistance between the conductive layer and the detection electrode or between the conductive layer and the reference electrode may be measured. The internal-resistance detection electrode may be formed separately from the reference electrode and the detection electrode (for example, as shown in FIGS. 6 and 9).
Preferably, as specified in a twelfth aspect of the invention, the solid electrolyte element assumes the form of a closed-bottomed cylinder; the heater element disposed within the solid electrolyte element assumes a rod form; the central axis of the solid electrolyte element and the central axis of the heater element substantially coincide with each other; and at least a portion of an end of the heater element is in contact with the inner surface of a bottom portion of the solid electrolyte element.
The above-mentioned xe2x80x9csubstantially coincide with each otherxe2x80x9d means that the solid electrolyte element and the heater element are coaxial or that the central axes are separated from each other by not more than 500 xcexcm (preferably, not more than 200 xcexcm, more preferably 50 xcexcm).
The above disposition of the heater element expands a portion of the surface of the solid electrolyte element which is maintained at high temperature (preferably, 400-650xc2x0 C., more preferably 450-600xc2x0 C., further preferably 480-590xc2x0 C.). Further, the high-temperature portion of small temperature variations can be expanded (where the difference between the maximum and minimum temperatures falls within 100xc2x0 C., more preferably within 80xc2x0 C., further preferably within 50xc2x0 C.). That is, even when the detection electrode is formed, the temperature dependence of the device for measuring combustible-gas concentration derived from temperature variations on the surface of the solid electrolyte element can be reduced.
In this portion of the surface of the solid electrolyte element of high temperature and uniform temperature distribution (not necessarily identical with the uniformly heated high-temperature portion), when the device for measuring combustible-gas concentration has a maximum surface temperature of 400-650xc2x0 C., the difference between average temperature as measured on the surface of the device at a position 1 mm from a bottom portion of the device toward a head portion of the device and average temperature as measured on the surface of the device at a position 5 mm from the bottom portion toward the head portion can be not greater than 50xc2x0 C. (preferably not greater than 40xc2x0 C., more preferably not greater than 30xc2x0 C.). When the device for measuring combustible-gas concentration has a maximum surface temperature of 400-650xc2x0 C., variations in maximum temperature as measured at four locations which are 90 degrees apart from each other around the device can be suppressed to not greater than 60xc2x0 C. (preferably not greater than 50xc2x0 C., further preferably 40xc2x0 C.).
The surface of the device for measuring combustible-gas concentration means such a surface as observed after removal of a mounting shell of metal. Specifically, the surface temperature of the device is the temperature of the surface of the diffusion layer when a diffusion layer is formed, or the temperature of the surface of the detection electrode or the solid electrolyte element when a diffusion layer is not formed.
For the devices of the first through eleventh aspects of the invention for measuring combustible-gas concentration, reducing dependence of sensor output on temperature is important. To this end, preferably, the detection electrode is formed at only the uniformly heated high-temperature portion. Further, temperature distribution in the uniformly heated high-temperature portion is preferably uniform. Accordingly, preferably, as specified in the twelfth aspect of the invention, the central axis of the solid electrolyte element and the central axis of the heater element coincide or substantially coincide with each other. This feature establishes uniform temperature distribution on the surface of the uniformly heated high-temperature portion of the device for measuring combustible-gas concentration.
Further, at least a portion of the end of the heater element is in contact with the inner surface of a bottom portion of the solid electrolyte element, thereby expanding uniform temperature distribution up to the end of the bottom portion of the device for measuring combustible-gas concentration. In order to attain such contact, an end portion of the heater element may be formed such that the diameter decreases toward the end of the heater element. For example, the end portion of the heater element may be rounded as is the inner surface of the bottom portion of the solid electrolyte element.
In the devices of the first through twelfth aspects of the invention for measuring combustible-gas concentration, as a result of the detection electrode assuming the above-described structure, offset can be reduced. However, oxygen concentration influences measurement by the device of the present invention for measuring combustible-gas concentration. Thus, preferably, in order to accurately detect and measure a combustible gas, sensor output is corrected for oxygen concentration through subtraction of sensor output for oxygen concentration.
Such correction may be performed by, for example, the following methods: a portion of the solid electrolyte element of the device for measuring combustible-gas concentration is used as an oxygen sensor for measuring oxygen concentration; an oxygen sensor for measuring oxygen concentration is disposed separately from the device for measuring combustible-gas concentration; and a reference electrode and a detection electrode are formed so as to be exposed to the same gas to be measured, thereby directly obtaining sensor output after sensor output for oxygen concentration is cancelled.
A device of a thirteenth aspect of the invention for measuring hydrocarbon-gas concentration comprises an oxygen-ion-conductive solid electrolyte element, a reference electrode and a detection electrode formed on the surface of the solid electrolyte element, and a heater element for heating the solid electrolyte element. The device is characterized in that the detection electrode contains gold and a metallic oxide.
Examples of the above-mentioned xe2x80x9chydrocarbon gasxe2x80x9d include hydrocarbons having 2-15 carbon atoms. The device of the present aspect of the invention for measuring hydrocarbon-gas concentration can sensitively detect hydrocarbons, particularly unsaturated hydrocarbons having 2-15 carbon atoms, and can accurately measure the concentration thereof. The above-mentioned xe2x80x9creference electrodexe2x80x9d is an electrode which is brought into contact with a reference gas or is placed in an oxygen atmosphere of constant pressure established by means of an oxygen pump, and/or which can indicate a lower electric potential than does the detection electrode, upon contact with a hydrocarbon gas component contained in a gas being measured. The xe2x80x9cdetection electrode,xe2x80x9d xe2x80x9cheater element,xe2x80x9d xe2x80x9csolid electrolyte element,xe2x80x9d and xe2x80x9cmetallic oxidexe2x80x9d are similar to those of the first aspect of the invention.
A device of a fourteenth aspect of the invention for measuring hydrocarbon-gas concentration is characterized in that the detection electrode comprises a first electrode layer formed on the surface of the solid electrolyte element and a second electrode layer formed on the surface of the first electrode layer; the first electrode layer containing at least one of platinum and gold; and the second electrode layer containing at least one of gold and a metallic oxide (except for a case where the first electrode layer is identical in composition with the second electrode layer).
A device of a fifteenth aspect of the invention for measuring hydrocarbon-gas concentration is characterized in that the detection electrode comprises a first electrode layer formed on the surface of the solid electrolyte element, a second electrode layer formed on the surface of the first electrode layer, and a third electrode layer formed on the surface of the second electrode; the first electrode layer containing a predominant amount of platinum; the second electrode layer containing a predominant amount of gold; and the third electrode layer containing a metallic oxide and optionally gold.
In the fourteenth and fifteenth aspects of the invention, xe2x80x9chydrocarbon gas,xe2x80x9d xe2x80x9creference electrode,xe2x80x9d xe2x80x9cdetection electrode,xe2x80x9d xe2x80x9cheater element,xe2x80x9d xe2x80x9csolid electrolyte element,xe2x80x9d and xe2x80x9cmetallic oxidexe2x80x9d are similar to those of the thirteenth aspect of the invention. In the fourteenth aspect of the invention, xe2x80x9cfirst electrode layerxe2x80x9d and xe2x80x9csecond electrode layerxe2x80x9d are similar to those of the second aspect of the invention. In the fifteenth aspect of the invention, xe2x80x9cfirst electrode layerxe2x80x9d and xe2x80x9csecond electrode layerxe2x80x9d are similar to those of the third aspect of the invention.
Preferably, as specified in a sixteenth aspect of the invention and a seventeenth aspect of the invention, as in the case of the fourth and fifth aspects of the invention, the detection electrode is formed at only a uniformly heated high-temperature portion.
A device of an eighteenth aspect of the invention for measuring hydrocarbon-gas concentration is characterized in that the detection electrode contains gold and optionally platinum and is formed on the solid electrolyte element at only a portion extending from an end portion of the solid electrolyte element to the vicinity of the interface between a heating resistor and a heating-resistor lead portion, which are formed within the heater element.
The above-mentioned xe2x80x9chydrocarbon gas,xe2x80x9d xe2x80x9creference electrode,xe2x80x9d xe2x80x9cdetection electrode,xe2x80x9d xe2x80x9cheater element,xe2x80x9d and xe2x80x9csolid electrolyte elementxe2x80x9d are similar to those of the thirteenth aspect of the invention. The above-mentioned xe2x80x9cvicinity of the interface between a heating resistor and a heating-resistor lead portionxe2x80x9d and xe2x80x9cend portion of the solid electrolyte elementxe2x80x9d are similar to those of the sixteenth aspect of the invention.
A device of a nineteenth aspect of the invention for measuring hydrocarbon-gas concentration is characterized in that the detection electrode comprises a first electrode layer formed on the surface of the solid electrolyte element and a second electrode layer formed on the surface of the first electrode layer; the first electrode layer containing platinum; the second electrode layer containing gold (except for a case where the first electrode layer is identical in composition with the second electrode layer); and the second electrode layer is formed from paste containing gold powder of an average grain size of 1-100 xcexcm (preferably 2-50 xcexcm, more preferably 5-30 xcexcm) and by baking the paste.
The above-mentioned xe2x80x9chydrocarbon gas,xe2x80x9d xe2x80x9creference electrode,xe2x80x9d xe2x80x9cdetection electrode,xe2x80x9d xe2x80x9cheater element,xe2x80x9d and xe2x80x9csolid electrolyte elementxe2x80x9d are similar to those of the thirteenth aspect of the invention. The average grain size of gold power is similar to that associated with the seventh aspect of the invention.
Preferably, as specified in a twentieth aspect of the invention, as in the case of the eighth aspect of the invention, a diffusion layer is formed on the surface of the detection electrode in any of the thirteenth through nineteenth aspects of the invention. Preferably, as specified in a twenty-first aspect of the invention, as in the case of the ninth aspect of the invention, the diffusion layer contains a component which oxidizes at least one of hydrogen and carbon monoxide. Particularly, as specified in a twenty-second aspect of the invention, as in the case of the tenth aspect of the invention, the diffusion layer contains at least one of platinum and palladium.
When the device for measuring hydrocarbon-gas concentration in which the diffusion layer contains platinum is applied to measurement of a gas to be measured which contains hydrogen; specifically, a gas to be measured which contains H2 (1000 ppm), O2 (7%), CO2 (10%), H2O (10%), and N2 (balance) and which has a temperature of 300xc2x0 C. and flows at 15 liters/min, and the device is controlled in the course of measurement such that the surface temperature of the detection electrode becomes 570xc2x0 C., sensor output for hydrogen can be reduced by 80 mV or more (approx. 25% or more) as compared with measurement by a device in which the diffusion layer does not contain platinum.
When the device for measuring hydrocarbon-gas concentration in which the diffusion layer contains platinum is applied to measurement of a gas to be measured which contains carbon monoxide; specifically, a gas to be measured which contains CO (1000 ppm), O2 (7%), CO2 (10%), H2O (10%), and N2 (balance) and which has a temperature of 300xc2x0 C. and flows at 15 liters/min, and the device is controlled in the course of measurement such that the surface temperature of the detection electrode becomes 570xc2x0 C., sensor output for carbon monoxide can be reduced by 80 mV or more (approx. 80% or more), further by 85 mV or more (approx. 85% or more), as compared with measurement by a device in which the diffusion layer does not contain platinum.
When the device for measuring hydrocarbon-gas concentration in which the diffusion layer contains palladium is applied to measurement of a gas to be measured which contains carbon monoxide; specifically, a gas to be measured which contains propylene (1000 ppmC) or CO (1000 ppm), O2 (7%), CO2 (10%), H2O (10%), and N2 (balance) and which has a temperature of 300xc2x0 C. and flows at 15 liters/min, and the device is controlled in the course of measurement such that the surface temperature of the detection electrode becomes approximately 570xc2x0 C., sensor output for carbon monoxide can be reduced by 14 mV or more (approx. 15% or more), further by 20 mV or more (approx. 20% or more), as compared with measurement by a device in which the diffusion layer does not contain palladium. Further, this device for measuring hydrocarbon-gas concentration can improve sensor output for propylene by 15 mV or more (approx. 13% or more), further by 20 mV or more (approx. 16% or more).
Preferably, as specified in a twenty-third aspect of the invention, as in the case of the eleventh aspect of the invention, the device of the present invention for measuring hydrocarbon-gas concentration further comprises temperature control means for controlling voltage applied to the heater element on the basis of the internal resistance of the solid electrolyte element. Preferably, as specified in a twenty-fourth aspect of the invention, as in the case of the twelfth aspect of the invention, the solid electrolyte element assumes the form of a closed-bottomed cylinder; the heater element disposed within the solid electrolyte element assumes a rod form; the central axis of the solid electrolyte element and the central axis of the heater element substantially coincide with each other; and at least a portion of an end of the heater element is in contact with the inner surface of a bottom portion of the solid electrolyte element. Preferably, in the devices of the thirteenth through twenty-fourth aspects of the invention for measuring hydrocarbon-gas concentration, as in the case of the previously-described devices for measuring combustible-gas concentration, sensor output is corrected for oxygen concentration through subtraction of sensor output for oxygen concentration.
The invention also provides, as twenty-fifth and twenty-sixth aspects, methods for measuring combustible-gas concentration or for measuring hydrocarbon-gas concentration, using a device according to one of the first through twelfth aspects of the invention or thirteenth through twenty-fourth aspects of the invention, respectively, wherein the internal resistance of the solid electrolyte is measured periodically, and a voltage applied to the heater element is controlled such that the internal resistance becomes constant.
Note that throughout this specification, the terms xe2x80x9cpropenexe2x80x9d and xe2x80x9cpropylenexe2x80x9d are used synonymously. Propene is the more strictly correct term, whereas propylene is the term commonly used in this field.